Reads all legacy keys (sprint_pb_*, pbi_*, story_sort, debug-mode,
and dynamic *_filter_kind/*_filter_status for jobs columns) and
returns a typed UserSettings patch plus the keys to clear.
Idempotent via scrum4me:settings_migrated=v1 marker. Skips invalid
values silently so existing corrupt entries do not block migration.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
